System and method for sub micron additive manufacturing

ABSTRACT

An apparatus is disclosed for performing an additive manufacturing operation to form a structure by processing a photopolymer resist material. The apparatus may incorporate a laser for generating a laser beam, and a tunable mask for receiving the laser beam which has an optically dispersive element. The mask splits the laser beam into a plurality of emergent beams each having a subplurality of beamlets of varying or identical intensity, with each beamlet emerging from a unique subsection of illuminated regions of the mask. A collimator collimates at least one of the emergent beams to form a collimated beam. One or more focusing elements focuses the collimated beam into a focused beam which is projected as a focused image plane on or within the resist material. The focused beam simultaneously illuminates a layer of the resist material to process an entire layer in a parallel fashion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNo. PCT/US2017/059326, filed on Oct. 31, 2017. The entire disclosure ofthe above application is incorporated herein by reference.

STATEMENT OF GOVERNMENT RIGHTS

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

FIELD

The present disclosure relates generally to additive manufacturingsystems and methods, often referred to as 3D printing (three-dimensionalprinting), and more particularly to a method and apparatus for high-rateadditive manufacturing of structures with submicron features using amultiphoton, non-linear photo-absorption process, wherein the system andmethod enables fabrication of features that are smaller than thediffraction-limited focused illumination spots.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Two-photon polymerization, also sometimes referred to as two-photonlithography, is a popular present day technique to additivelymanufacture complex 3D structures with submicron building blocks. Thistechnique uses a nonlinear photo-absorption process to polymerizesubmicron features within the interior of the photopolymer resistmaterial. After illumination of the desired structures inside thephotoresist volume and subsequent development (washing out thenon-illuminated regions), the polymerized material remains in theprescribed three-dimensional form. One example of a system that may bemodified to use for two-photon polymerization is described in U.S.Patent Publication No. 2016/0199935 A1, published Jul. 14, 2016, theentire contents of which are hereby incorporated by reference into thepresent disclosure.

Two-photon polymerization is a direct-write technique that enablesfabrication of macroscale complex 3D structures with submicron features.In its most commonly implemented form, the writing of complex structuresis achieved through a serial writing technique wherein a high lightintensity spot is sequentially scanned in 3D space to generate theentire structure. Due to the serial writing scheme, the rate of writingis fundamentally limited to such an extent that two-photon lithographyof large volumes of functional parts is not feasible. Although attemptsto increase the rate via parallelization have been made in the past,such attempts have failed to achieve the same degree of patterncomplexity as that which can be accomplished with the point-scanningserial technique. Specifically, past parallelization efforts have eithergenerated arrays of identical features or have been used to print 2Dparts with no depth resolvability.

Although two-photon lithography enables fabricating features on a lengthscale that is not possible by other additive manufacturing techniques,the serial writing scheme that it uses limits this method to a lowprocessing rate of ˜0.1 mm³/hour. This prevents taking full advantage ofits submicron geometric control to fabricate functional parts. Technicaland scientific challenges in solving this low processing rate limitationarise because of the slow point-by-point serial illumination techniqueof the existing systems. The problem of performing parallel two-photonlithography (“TPL”) without adversely affecting the ability to fabricatearbitrarily complex 3D parts has not been solved in the past. Thefollowing two general approaches exist in the prior-art that partlysolve the problem of parallelization of TPL: (i) “splitting” a beam andsimultaneously focusing at multiple spots to fabricate identicalfeatures at multiple spots (see Vizsnyiczai, G., Kelemen, L., and Ormos,P., 2014, “Holographic multi-focus 3D two-photon polymerization withreal-time calculated holograms,” Opt. Express, 22(20), pp. 24217-24223),(ii) projecting an arbitrarily complex 2D image into the resist togenerate 2D structures without depth resolvability (see Mills, B.,Grant-Jacob, J. A., Feinaeugle, M., and Eason, R. W., 2013,“Single-pulse multiphoton polymerization of complex structures using adigital multimirror device,” Opt. Express, 21(12), pp. 14853-14858).

The first approach is unsuitable for TPL scale up because, in thisapproach, scale-up is achieved by simultaneously printing a structureover multiple spots in the form of a periodic array. As the same beam issplit into multiple identical beams, identical features are generated byeach beam. Thus, no scale-up is achieved during printing of arbitrarilycomplex non-periodic structures using this technique.

The second approach is unsuitable for printing of complex 3D structuresbecause depth resolvability is lost in these projection techniques.Depth resolvability refers to the ability to process a thincross-section of the resist material without processing anything belowor above the processed cross-section. For submicron additivemanufacturing, depth resolution (i.e., thickness of processedcross-section of resist) ranging from less than 1 μm to a few microns isdesirable. However, in this second approach, when a 2D image isprojected through the resist material, a single focal planeperpendicular to the 2D projected image cannot be uniquely registered.Instead, the same 2D image is “focused” at multiple planes so that athick 3D cured volume is generated in the form of an extrusion of the 2Dimage throughout the thickness of the resist layer. Thus, this schemecannot be used to print 3D structures with depth resolved features suchas those present in 3-D truss structures.

Temporal focusing of wideband femtosecond laser sources has beenpreviously applied for fluorescence imaging of biomaterials. Thistechnique has also been used to demonstrate material removal basedfabrication processes. It has been suggested that such temporal focusingsystems can also be used for multiphoton lithography. However, theseteachings fail to enable high-quality 3D printing of structures withoutundue experimentation. Underlying this failure is the key differencebetween the physical mechanism of multiphoton lithography (“MPL”) andthat of imaging or material removal. Specifically, the dosage thresholdbehavior of resists used during MPL is distinct from that of materialremoval or imaging processes. In imaging and material removal, exposuredosage refers to the time-integrated photon energy; this is because theunderlying physical processes are driven by the total amount of energy(dosage˜intensity×time). In contrast, the exposure dosage during MPLnonlinearly combines the light intensity and the exposure time(dosage˜(intensity)^(a)×(time)^(b), where ‘a’ and ‘b’ are real positivenumbers). This nonlinear form for the exposure dosage arises due to acombination of the nonlinear photo-absorption process and kinetics ofthe chemical reactions underlying the polymerization process. As aresult of this, prior art techniques that achieve dosage control bytime-averaging the light intensity are inappropriate for nonlineardosage control in MPL. If such techniques are used in MPL, either blobsof overexposed structures are generated or structures with underexposedregions are obtained. Herein, the tools and techniques for appropriatedosage control in parallelized MPL are presented.

Accordingly, the needs still exist for a system and method which is ableto dramatically increase the rate of two-photon lithography withoutadversely affecting the ability to fabricate arbitrarily complex 3Dstructures.

SUMMARY

In one aspect the present disclosure relates to an apparatus forperforming an additive manufacturing operation to form a structure byprocessing a photopolymer resist material. The apparatus may comprise alaser source for generating a laser beam and a tunable mask forreceiving the laser beam. The tunable mask may comprise an opticallydispersive element. The tunable mask may be configured to split thelaser beam into a plurality of emergent beams, wherein each emergentbeam emerging from the tunable mask comprises a subplurality of beamletsof varying or identical intensity, and wherein each beamlet emerges froma unique subsection of illuminated regions of the tunable mask. Acollimator may be included for collecting and collimating at least oneof the emergent beams from the tunable mask to form a collimated beam.One or more focusing elements may be included to focus the collimatedbeam into a focused beam which is projected as a focused image planeonto or within the photopolymer resist material. The tunable mask, thecollimator and the focusing elements are so oriented and positioned asto create the same optical path length between the tunable mask and theimage plane for all optical frequencies of the laser beam. The focusedbeam simultaneously illuminates a layer of the photopolymer resistmaterial.

In another aspect the present disclosure relates to an apparatus forperforming an additive manufacturing operation to form a structure byprocessing a photopolymer resist material. The apparatus may comprise alaser source for generating a pulsed laser beam and a tunable mask. Thetunable mask may be used for receiving the pulsed laser beam, andincludes a digital micromirror device (DMD) having a plurality ofindependently controllable pixels that may be turned on or off. Thetunable mask may be configured to split the pulsed laser beam into aplurality of emergent beams, wherein each emergent beam emerging fromthe tunable mask comprises a subplurality of beamlets of varying oridentical intensity, and wherein each beamlet emerges from a uniquepixel. A collimator may be used for collecting and collimating allwavelengths of only a select one of the plurality of emergent beams fromthe tunable mask, and where the collimator is configured to block allwavelengths of all other ones of the emergent beams. One or morefocusing elements may be used to focus the collimated beam as a focusedbeam into a focused image plane on or within the photopolymer resistmaterial. The tunable mask, the collimator and the focusing elements areso oriented and positioned as to create the same optical path lengthbetween the tunable mask and the focused image plane for all opticalwavelengths of the pulsed laser beam. A motion stage may be included tosupport and move the photopolymer resist material relative to thefocused image plane. The focused beam simultaneously illuminates a layerof the photopolymer resist material.

In still another aspect the present disclosure relates to a method forperforming an additive manufacturing operation to form a structure byprocessing a photopolymer resist material. The method may comprisegenerating a laser beam, and directing the laser beam at a tunable mask,wherein the tunable mask comprises an optically dispersive element. Themethod may further include collecting at least one emergent beam from aplurality of emergent beams emerging from the tunable mask and directingit through a collimating optics to generate a collimated beam. Eachemergent beam from the tunable mask comprises a plurality of beamlets ofvarying or identical intensity, and each beamlet emerges from a uniquesubsection or region of the tunable mask being illuminated by the laserbeam. The method may further involve focusing the collimated beamthrough one or more focusing elements into a focused beam which isprojected as an image plane onto or within the photopolymer resistmaterial, wherein the tunable mask, the collimating optics, and thefocusing elements are so oriented and positioned as to create the sameoptical path length between the tunable mask and the focused image planefor all optical frequencies of the laser beam. The method may furtherinvolve holding a select pattern of subsections on the tunable mask fora finite duration of time to cause simultaneous polymerization of selectportions of the photopolymer resist material corresponding to the selectpattern, at which the combined dosage effect from a duration of laserillumination and an intensity of light exceed a threshold dosage forpolymerization of the photopolymer resist material.

In still another aspect the present disclosure relates to a method forperforming an additive manufacturing operation to form a structure byprocessing a photopolymer resist material. The method may comprisegenerating a pulsed laser beam and directing the pulsed laser beam at atunable mask, wherein the tunable mask is an optically dispersiveelement. The method may further involve collecting at least one emergentbeam emerging from the tunable mask and directing it through acollimating optics to generate a collimated beam. Each emergent beamfrom the tunable mask comprises a plurality of beamlets of varying oridentical intensity, and each beamlet emerges from a unique subsectionof the illuminated regions of the tunable mask. The method may furtherinvolve focusing the collimated beam through one or more focusingelements onto an X-Y image plane on or within the photopolymer resistmaterial. The tunable mask, the collimating optics, and the focusingelements are so oriented and positioned as to create the same opticalpath length between the tunable mask and the image plane for all opticalfrequencies of the laser beam. The plurality of beamlets are able tosimultaneously illuminate a layer of the photopolymer resist material.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a high level block diagram of an apparatus in accordance withone embodiment of the present disclosure for performing an additivemanufacturing operation to produce a sample part having submicronfeatures;

FIG. 2 is a high level block diagram of another embodiment of thepresent disclosure, somewhat similar to the apparatus of FIG. 1, butwhich also makes use of a power meter and a beam power control unit forcontrolling a power of a processing beam in real time;

FIG. 3 is a high level block diagram of another embodiment of thepresent disclosure, that encapsulates key functions into sub-systems;

FIG. 4 is a graph illustrating the empirically derived dosage law inparallel two-photon lithography for a particular polymeric materialbeing processed and at differing feature spacings;

FIGS. 5a and 5b illustrate exemplary patterns of DMD “on” states (i.e.,bright regions) for grayscale control, with FIG. 5a illustrating apattern with both vertical bright bars and horizontal bright bars, FIG.5b illustrating a pattern with only horizontal bright bars. FIG. 5cillustrates the desired printed pattern in the photoresist materialcorresponding to the DMD patterns of FIGS. 5a and 5 b;

FIG. 5c is an isometric illustration of an actual part produced usingthe masks of FIGS. 5a and 5 b;

FIG. 5d illustrates an image obtained using a scanning electronmicroscope of a part in the foreground which has been manufactured usingthe teachings of the present disclosure, and illustrating the depthresolvability of the present disclosure to resolve individual Z planesof the part;

FIG. 6 is a high level flowchart illustrating basic operations that maybe performed using the methodology of the present disclosure; and

FIG. 7 is a high level flowchart illustrating basic operations that maybe performed using the methodology of the present disclosure, with thesteps that enable grayscale and super-resolution printing.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

The present disclosure overcomes the above-described manufacturing ratelimitations with two-photon lithography with a system and method whichimplements a parallel illumination technique. The parallel illuminationtechnique simultaneously projects an entire plane of ˜1 million points,rather than the single-point illumination technique of existingcommercial systems to increase the rate by at least 100 times.

The present disclosure is also distinguished from other popular 3Dprinting techniques (commonly referred to as projectionmicro-stereolithography) through its use of high peak-power pulsed lasersources, and its ability to generate features in the interior ofmaterials being processed (“resist material”). High peak-power pulsedsources are used in this technique to ensure that nonlinearphoto-absorption is observed in exclusion to single-photon linearphoto-absorption. Photopolymer resist materials may be selected suchthat they exhibit a “thresholding” behavior, i.e., they undergo a phasetransition (commonly from liquid to solid) upon exposure to light onlywhen the nonlinear light exposure dosage exceeds a minimum thresholdvalue (called the “threshold dosage”). Resist materials in which thelight-exposed regions become more soluble in solvents may also be used.As the dosage in multiphoton lithography scales nonlinearly with thelight intensity in the exposed material, steeper dosage gradients can begenerated in the material during nonlinear absorption than in linearabsorption. This steeper dosage gradient leads to a processed featurethat is smaller than the diffraction-limited illumination spot; thesteeper gradient also enables generation of individual, spot-like voxelfeatures (or volumetric pixels) within the interior of the resistmaterial by focusing a laser spot at an interior spot. Thus, the presentapparatus and method differs from conventional micro-stereolithographyin both form and function. The form differs in the use of pulsed lasersources (with the present apparatus and method, during multiphotonlithography) versus incoherent sources (in conventionalmicro-stereolithography); whereas, the function differs in the abilityto fabricate submicron features in the interior of the resist (i.e., thepresent apparatus and method) vs diffraction-limited features on thesurface of the resist (as with conventional micro-stereolithography).

Within the area of multiphoton lithography (MPL), the apparatus andmethod of the present disclosure is distinguished from existingimplementations in its ability to simultaneously focus a collection ofpoints (i.e., focus a “projected image”) in the interior of the resistmaterial without providing significant light intensity above or belowthe focused depth. Thus, this technique significantly increases theprocessing rate by parallelizing the generation of submicron features.It is important to note that with the apparatus and method describedherein, the dosage at each individual focused spot may be independentlytuned to generate arbitrarily complex patterns. Thus, the apparatus andmethod of the present disclosure differs from, and significantlyimproves upon, previously existing MPL implementations that split thesame beam into multiple focal spots with identical intensitydistribution. In addition, by incorporating features for nonlineardosage gradients, the apparatus and method of the present disclosurediffers from, and significantly improves upon, those previously existingmulti-point MPL implementations that fail to preserve the steep dosagegradients experienced during single-point focusing. With the presentapparatus and method, steep spatial gradients in the dosage are achievedby taking advantage of the time-dependence of intensity in beams thatare generated by pulsed laser sources. Specifically, steep dosagegradients are achieved through temporal focusing of wideband femtosecondpulsed lasers.

The apparatus and method of the present disclosure takes advantage of atechnique known as “temporal focusing.” Temporal focusing refers to thephenomenon wherein the duration of a femtosecond pulse (nominally 100 fsor less) is progressively shortened in conjunction with spatial focusingof the beam. As the peak-intensity during a pulse depends on both thespatial size of the beam and the duration of the pulse, the intensitycan be independently tuned by changing either of these two. In serialpoint scan implementations, focusing of the beam to a single point isachieved by only tuning the spatial size of the beam without any tuningof the duration of the pulse. In contrast, the apparatus and method ofthe present disclosure implements an optical projection scheme whereinthe pulse duration is progressively reduced in proportion to the size ofthe beam such that the beam is both spatially and temporally focused onor within the interior of the resist material and wherein the locationof spatial and temporal focused spots overlaps. This ensures that steepdosage gradients are achieved at the projected image plane even when theprojected image is large (due to multiple focused spots). It isimportant to note that this projection scheme differs in form andfunction from that of conventional projection micro-stereolithographyimplementations due to the reliance on temporal properties of thewideband femtosecond pulsed laser source to achieve the focusingdescribed herein. An important element of the temporal focusingtechnique is that the optical path lengths after the projection mask aredesigned to match for all optical frequencies in the beam only at thefocused image plane but mismatched at all other planes. Thus, temporalfocusing stretches the pulse by introducing a “chirp” and selectivelyminimizes (and ideally eliminates) this chirp only at the focused imageplane.

Temporal focusing of wideband femtosecond laser sources has beenpreviously applied for fluorescence imaging of biomaterials. Thistechnique has also been used to demonstrate material removal basedfabrication processes. It has been suggested that such temporal focusingsystems can also be used for multiphoton lithography. However, theseteachings fail to enable high-quality 3D printing of structures withoutundue experimentation. Underlying this failure is the key differencebetween the physical mechanism of multiphoton lithography (MPL) and thatof imaging or material removal. Specifically, the dosage thresholdbehavior of resists used during MPL is distinct from that of materialremoval or imaging processes. In imaging and material removal, exposuredosage refers to the integrated photon energy; this is because theunderlying physical processes are driven by the total amount of energy(dosage˜intensity×time). In contrast, the exposure dosage during MPLnonlinearly combines the light intensity and the exposure time(dosage˜(intensity)^(a)×(time)^(b), where ‘a’ and ‘b’ are real positivenumbers). As a result of this, prior art techniques that achieve dosagecontrol by time averaging the light intensity are inappropriate fordosage control in MPL. If such techniques are used in MPL, either blobsof overexposed structures are generated or structures with underexposedregions are obtained. Herein, the tools and techniques for appropriatedosage control in parallelized MPL are presented.

It is important to note that the nonlinear dosage characteristic alsoaffects proper selection of the laser source. As the threshold dosage isdetermined by the peak intensity of light (i.e., maximum instantaneousintensity), the peak intensity for all focused spots in the parallelscheme must be similar to that in the serial scanning technique. Inserial scanning techniques, this focused intensity lies in the range of0.1 to about 2 TW/cm². This suggests that for a field of view of a fewhundreds of microns (i.e., focused beam size), the peak beam powershould be in the range of ˜1 GW. High repetition rate (>10 s of MHz)femtosecond laser oscillators and low repetition rate (˜1 to 10 kHz)femtosecond laser amplifiers are among the potential choices for pulsedlaser sources. Despite having similar average powers, laser amplifierswith high peak powers are preferred sources for parallel submicronadditive manufacturing due to their 4 to 5 orders of magnitude (i.e.,10,000 to 100,000 times) higher peak powers.

FIG. 1 shows an apparatus 10 in accordance with one embodiment of thepresent disclosure. The apparatus 10 may include a pulsed laser sourcein the form of a laser amplifier 12, an optical parametric amplifier(“OPA”) 14, a beam expander 16, a first high reflective (“HR”) mirror18, a beam homogenizer 20, a second HR mirror 22, a second beam expander24, a tunable mask 26, an electronic digital mask control system 27(which includes processor, memory and I/O), a concave mirror 30, aneutral density (“ND”) filter 32, a short-pass filter (“SF”) 36, acharge coupled display (“CCD”) camera 40 an electrically tunable lens(“ETL”) 42, an objective lens 44, a sample (comprising “photopolymerresist material” supported on optically clear slide) 48, a movable stage50 and a lamp 52 projecting a beam 54 toward the sample 48 for imagingof the processing zone. This lamp 52 may be an incoherent light sourceof such a wavelength spectrum that does not affect photo-polymerizationof the resist material.

In operation, the laser source 12 may be a pulsed laser source thatprovides the laser light that drives the writing process. A key featureof this laser source 12 is that it generates pulses with a broadwavelength spectrum instead of a single wavelength. One example of asuitable laser source is a femtosecond Ti-sapphire regenerative laseramplifier with a center wavelength of 800 nm, a pulse duration of 35 fsand a bandwidth of 40 nm. As shown in FIG. 1, light from the lasersource 12 has its wavelength modified by the OPA 14, in one example from800 nm to either 325 nm or 500 nm, before being further modified by thefirst beam expander 16, the beam homogenizer 20 and the second beamexpander 24. The beam homogenizer modifies the shape of the beam from anon-uniform Gaussian profile to a uniform flat-top beam profile. Beamexpanders 16 and 24 control the diameter of a beam 25 that illuminatesthe digital mask 26. Multiple beam expanders may be required toindividually match the size of the beam to apertures of the variouscomponents such as the beam homogenizer and the tunable mask.

The tunable mask 26, in one example, may be a digital micro-mirrordevice (“DMD”). This component is commercially available from variousmanufacturers, for example Texas Instruments Inc. of Dallas, Tex.Alternatively, the tunable mask 26 may be formed by a spatial lightmodulator (SLM). The tunable mask may also be a strain-driven tunablediffraction grating such as those formed by wrinkling of supported thinfilms (e.g., see S. K. Saha and M. L. Culpepper, Biaxial Tensile Stagefor Fabricating and Tuning Wrinkles, U.S. Pat. No. 9,597,833 B2, March2017, hereby incorporated by reference into the present disclosure) or afixed uniform or non-uniform grating mounted on a movable (rotatingand/or translating) mount. The key feature of this tunable mask 26 thatenables temporal focusing is that it is a dispersive optical element,i.e., it is capable of spatially separating the different opticalfrequencies (or wavelengths) of the incident beam. DMD, SLM, and tunablewrinkled films can all act as dispersive elements due to their periodicstructure which diffracts light. When a laser source is incident ontosuch a dispersive element, it diffracts into multiple beams. The angularposition of the diffracted beams is determined by the modes ofdiffraction. Each of these diffracted beams contains the fullinformation about illuminated patterned subsections of the tunable maskin the form of individual beamlets that correspond to these subsections.These patterned subsections may correspond to individual mirrors in theDMD or individual peaks of a wrinkled grating wherein these subsectionsare themselves tunable. If the laser source is a broadband source, thebeamlets (and the beams) emerging from the mask diverge instead of beingin the form of a single beamlet or beam. This is because the angularposition of the diffracted beamlets (and beams) is dependent on thespecific wavelength. Due to the inverse relationship between spectralbandwidth and duration of pulse, this spatial divergence (induced by thedispersive mask) stretches the pulse duration and is a key feature thatensures temporal focusing. Tunability of the mask ensures thatstructures with various feature geometries can be printed. For thefollowing discussion, it will be assumed that the tunable mask 26 isformed by a DMD.

With a DMD used as the tunable mask 26, a key feature is that each micromirror of the DMD may be viewed as forming a pixel point, and each pixelpoint can be individually switched on or off. This is accomplished byrotating the mirror by a small angular amount between two predeterminedpositions (often +or −12 degrees). In one predetermined position thepixel (i.e., micro mirror) forms an “on” state where the intensity oflight emerging from the micro mirror pixel via reflection anddiffraction along a particular set of directions is high whereas theother predetermined position forms an ‘off’ state where the intensity ofthe emergent light along the same set of directions is zero or a lowvalue. Here, the cutoff for the qualitative terms ‘high’ and ‘low’ isdetermined by the specific downstream application. Often, commerciallyavailable DMD systems are designed so that the ratio of intensity forthe off versus on state along a particular propagation direction isalmost zero for incoherent light. This illumination tuning issufficiently high for two-photon lithography to achieve two differentexposure states (high exposure versus zero exposure). For the highexposure state, exposure of the resist through a finite number of laserpulses is sufficient to affect polymerization. For the low exposurestate, the intensity is too low to affect polymerization even with aninfinitely large number of pulses due to the thresholding behavior ofpolymeric resist materials. The beamlets created from each of the “on”micro mirrors in the tunable mask 26 form a diverging beam which isdenoted by reference number 28 and collectively form an image createdusing the tunable mask 26. Therefore, only the beam lets created from an“on” pixel within the tunable mask 26 are used to affect polymerizationof material within the sample 48. The rest of the beamlets (i.e., allbeamlets emerging from the ‘off’ pixels) may be re-directed into one ormore light sinks. It is important to note that several diffracted beamsemerge from the mask 26 and each of these diffracted beams comprises the“on” beamlets. These beams differ in the angular position (diffractedmode) and energy (mode efficiency). For polymerization, it is preferableto use the beam from only that mode which has the highest diffractionefficiency (i.e., the highest energy). Other modes (beams) may be usedfor diagnostics or re-directed into light sinks. The additionaldiffracted beams are not shown in FIG. 1 but one additional diffractedbeam is shown in FIG. 2. The collimating optics (i.e., concave mirror30) converts the diverging beam 28 into a collimated beam 34. While thecollimating optics in this example is shown as the concave mirror 30, asuitable lens could also be used to provide the needed collimation ofthe diverging beam 28.

The collimated beam 34 then passes through the neutral density ND filter32, through the short-pass filter 36, through the ETL 42, the objectivelens 44, and is focused onto an X-Y plane inside the sample 48, asindicated by focused beam 46. The sample 48, as noted above, maycomprise a photopolymer resist material. This focused X-Y plane is theconjugate plane of the tunable mask 26. An example of an objective lenssuitable for use as the object lens 44 is a high numerical aperture butlow to moderate magnification oil immersion infinity objective lens(such as a 40× 1.4 NA lens).

At the focused image plane on or within the sample 48, when the laserillumination is held for a finite duration of time, the exposure dosageat each point in the resist that corresponds to the “on” pixel in thetunable mask 26 is higher than the threshold dosage of the materialcomprising the sample 48; whereas, the exposure dosage at each point inthe resist corresponding to an “off” pixel is below the thresholdexposure dosage. Thus, a pixelated image of the tunable mask 26 isformed in an X-Y plane within the sample 48. This enables an entirelayer within the sample 48 to be written out in one operation, as thebeamlets of the beam 34 from “on” pixels are able to simultaneouslywrite, in parallel, to a large plurality of points (i.e., on the orderof 1×10⁶ or more) in one operation. Thus, the ability to form each layerof the sample 48 with a plurality of beamlets that write in parallelenables a dramatic reduction in the time needed to create a finishedpart from the photopolymer resist material of the sample 48.

Three-dimensional structures may be fabricated by moving the focusedimage plane relative to the sample 48 using the movable stage 50. Inactual practice, the motion of the movable stage 50 in the X-Y-Z planemay be controlled by an electronic control system 56. Alternatively, themovable stage 50 could be a fixedly supported stage (i.e., not movable)while the objective lens 44 is moved within a Z plane as needed. Stillfurther, possibly both the movable stage 50 and the objective lens 44could be moved simultaneously. However, it is anticipated that for themajority of applications, it will be preferred to move only one or theother of the movable stage 50 or the objective lens 44. Further, thefocal plane of the objective lens 44 may be optically scanned in theaxial Z (i.e., depth into the sample) direction using the electricallytunable lens (ETL) 42. The ETL provides the ability to rapidly move thefinal temporally-focused image plane without any mechanical movement ofthe objective lens or the movable stage, thereby leading to an increasein rate by as much as a factor of 10. More complex part geometries maybe generated by replacing the movable X-Y-Z stage with a 6-axis movablestage that is capable of motions along all six degrees of freedom (i.e.,capable of X, Y, Z translations and tip, tilt, and rotation angulardisplacements).

With the apparatus 10 described above, important features are thus theconditioning of the laser light from the laser source 12, the tunablemask 26, the axis of the concave mirror 30 (i.e., the collimatingoptics), and the relative size and position of the collimating optics(summarized in FIG. 3). The size and position of the collimating opticsis relevant to the 3D printing processing capability because of thediverging and multi-beam nature of the beams emerging from the tunablemask. For the purpose of temporal focusing, all portions of at least onediverging beam (i.e., one diffracted mode) must be collected by thecollimating optics. The gradient of light intensity at the focused imageplane reduces if only a part of a diffracted mode is collected. Thisreduced intensity gradient will lead to a loss of depth resolvabilityduring printing. In addition, partial sections of beams from otherdiffracted modes should be blocked to minimize intensity variations inthe projected image. In addition, to increase the optical transmissionefficiency the three elements (laser light, mask, collimating optics)are arranged such that a blazed grating condition is obtained for thetunable mask 26. A blazed grating condition requires that the light isincident on the blazed tunable mask 26 at a specific angle that isdetermined by the pixel spacing on the tunable mask, the centerwavelength of the laser light beam and the blaze angle of the grating(i.e., the angle by which the mirrors in the DMD are turned). Anexplanation of the blazed grating condition is provided in thereferenced article by Gu, C., Zhang, D., Wang, D., Yam, Y., Li, C., andChen, S.-C., 2017, “Parallel femtosecond laser light sheetmicro-manufacturing based on temporal focusing,” Precision Engineering,50, pp. 198-203, the entire contents of which are incorporated into thepresent disclosure by reference.

To ensure high-quality printing, the concave mirror 30 (i.e.,collimating optics) is arranged such that only the diffracted order thatcorresponds to the blaze condition is collected. Typically, thisrequires one to place the concave mirror 30 (i.e., collimating optics)at a predetermined angle to the face of the tunable mask 26 and to blockthe other orders by introducing apertures. This sets a condition on themaximum angular aperture. Additionally, one must ensure that all of thewavelengths (within the bandwidth of the laser source 12) are collectedby the concave mirror 30 (i.e., the collimating lens). As the differentwavelengths emerge at (slightly) different angles, this condition sets aminimum angular aperture for the collimating optics. Thus, the angularaperture over which the beamlets of the beam 28 must be collected ontothe concave mirror 30 (i.e., collimating optics) must lie within a smallband. Outside this band, the performance of the apparatus 10 may dropsignificantly to such an extent that depth resolvability for 3D printingis lost. Although this additional aperture-based design feature mayappear an obvious design goal in light of this disclosure, past attemptsat parallelizing multiphoton lithography using similar opticalconfigurations have failed to demonstrate 3D depth resolvability (seeMills et al., supra) thereby suggesting that designing and configuringan optical system that is capable of depth-resolved parallel multiphotonlithography is non-trivial even when the system uses known components.It is important to note that these past attempts have been successful inmelting-based machining operations despite failing in implementingpolymerization-based depth-resolved multiphoton lithography. Thus,success in thermally-driven machining processes does not automaticallyguarantee that the underlying system could also successfully 3D printdepth-resolved polymeric structures.

The apparatus 10 also facilitates the implementation of a grayscaleprinting method that ensures that high-quality parts can be fabricated.The grayscale printing method comprises the sequence of operations andthe selection of writing conditions in these operations that leads to anon-uniform “dosage” during printing within the same projected imageplane. The term “dosage” refers to the combined nonlinear effect oflight intensity and duration of light exposure (in the formdosage˜(intensity)^(a)×(time)^(b), where ‘a’ and ‘b’ are real positivenumbers). Writing occurs at a point when the dosage at that point isabove a threshold value (“threshold dosage”) for a given photopolymerresist material. For writing, a pixel must be continuously switched “on”for a duration of time that is longer than the threshold exposure time(represented by vertical axis in FIG. 4) at the incident light intensity(represented by horizontal axis in FIG. 4). Non-uniform dosage can beachieved by selectively switching some pixels on or off to selectivelyincrease or decrease the nonlinear dosage within the plane of the resistmaterial. Practically, this can be achieved by sending a series ofpatterns (i.e., map of pixel “on” and “off” states) to the DMD andholding each of the patterns for finite durations of time. These patternillumination durations would then be shorter than the maximum exposuretime required for any spot within the field of projection. The netnonlinear dosage at any point within the resist material is thecumulative combined dosage from each projected image. Here, the field ofprojection refers to the maximum area of any focused image that can beprojected onto/into the resist material. Thus, one may need to project aseries of non-intuitive DMD patterns to print the desired structure atthe focused plane through a process of sequentially projecting severalDMD patterns and nonlinearly and cumulatively combining the effect ofillumination intensity and duration of illumination.

In determining the range over which the net dosage can be tuned throughthis method, one must account for the shortest duration that a pixel canbe switched on and the rate at which the power of the incident beam canbe tuned. The shortest duration that a pixel can be switched “on” isdetermined by the pulse repetition rate of the laser source 12.Grayscale control enables tuning the total exposure time of each pixelbetween zero and the maximum required duration in steps of thereciprocal of pulse repetition rate. For example, if a projection fieldrequires a maximum of 20 pulses, then with a laser source at a pulserepetition rate of 1 kHz the exposure time can be discretely tunedbetween 0 and 20 ms in steps of 1 ms. This is achieved by loading a newsingle-bit image onto the DMD 26 every 1 ms. The dosage can be furthertuned if a high-speed (i.e., faster than ˜10 ms response time) beampower control unit is incorporated into the system. Without thisadditional power control unit, the dosage can be discretely controlledover several grayscale levels that are separated by the reciprocal ofpulse repetition rate. Beam power control would enable finer dosagecontrol than this. It is important to note that the grayscale method fordosage control disclosed here is distinct from the intensity controlschemes of commercial DMD masks (i.e., projectors). In commercial DMDprojectors, time-averaged intensity of a pixel can be controlled overseveral levels by changing the ratio of rate at which the mirrors areswitched between on and off states while the mirrors are continuouslycycled between on and off states.

It has also been experimentally observed that the threshold dosagedepends on the proximity of features within the sample 48 as illustratedin FIG. 4. This figure shows the minimum threshold exposure timerequired to affect polymerization in the resist material whenilluminated with specific peak intensity and for different featurespacings. In a sample part that contains closely spaced and sparselyspaced features, providing a uniform dosage leads to over or underexposure based defects. The grayscale dosage control of the grayscaleprinting method mentioned above allows for non-uniform control of dosagein the same focused plane. This is implemented by taking advantage ofthe pulsed nature of the laser light from the laser source 12 and themulti-pulse exposure threshold behavior of the material that makes upthe sample 48. Traditionally, one would keep projecting one image perfocused plane within the sample 48 until the desired uniform dosage isobtained at the particular plane within the sample. But with the presentmethod, an important distinction is that instead of projecting the sameimage, multiple images may be sequentially projected at the same imageplane. The digital image being created is altered so that the pixels ofthe tunable mask 26 at which the local dosage exceeds the non-uniformdosage threshold are switched off in the subsequent images. This tuningof the sequential images at the same image plane in the sample 48 mayeither be performed by prior experimental calibration of dosage laws(such as in FIG. 4) or in real-time by optically sensing the curingprocess through changes in contrast of images captured by the real-timeimaging system (such as the imaging sub-system 118′ in FIG. 3). Theoptical sensing system comprises a separate illumination lamp 52 butshares the same focusing elements 42 and 44 as the processing system togenerate an image of the processing plane on the camera. This enableslive imaging and recording of the printing process. To ensure that theoptical sensing/visualization system does not interfere with theprinting process, the wavelength of illumination in this system isselected to lie outside the absorption spectrum of the resist material.

Thus, the grayscale dosage control technique described herein, asimplemented by producing a plurality of sequential images at the sameimage plane, but with different dosages for each pixel of the image,enables printing of non-uniform parts without generating defects due toover or under-exposure. In particular, the printing of non-uniform partshaving closely spaced but differing features, is now possible.Representative grayscale digital masks (pattern of DMD pixel “on”states) are shown in FIGS. 5a and 5b that are required to print thestructure of FIG. 5c . FIG. 5D is a scanning electron microscope imageobtained of an actual part manufactured using the teachings of thepresent disclosure, as shown in the foreground image. The foregroundimage has been toppled by rotating it about its Y-axis edge on abottommost plane. The foreground image illustrates the individual depthresolved Z planes of a printed pillar structure, while the backgroundimage is shown in the “as-printed” upright orientation.

It should also be noted that time-averaged intensity is generally not areliable measure of the exposure dosage during multiphoton lithography.As a consequence, commercial intensity control techniques that rely ontime-averaging the intensity cannot be used for reliable dosage control.In addition, the grayscale technique presented, while being used to tunethe total exposure time, is not capable of tuning the instantaneous orpeak intensity. Although the intensity can be tuned by controlling thenet power of the incident beam, feedback for such tuning is often notavailable in real time. As the diffraction efficiency of the DMD 26 isprone to change with the spatial frequency of the image, a one-timecalibration of the transmitted power is not accurate for all images.This issue has been solved with another embodiment of the presentdisclosure which is shown in FIG. 2 as apparatus 100. The apparatus 100is somewhat similar to the apparatus 10 in that it includes a pulsedlaser source 102, a half wave plate 104, a polarizing beam splitter 106,a mirror 108, a tunable mask 110, an electronic control system 112 forcontrolling the tunable mask 110, a pair of collimating lenses 114 and115 each used to collimate the beam it receives, a beam monitoring powermeter 116, a beam power control unit 117, a camera 118 (the electroniccontrol system 112 also controlling the camera 118 in this example), alens 120, a beam splitter 122, a lamp 124, a dichroic mirror 126, anobjective lens 128, a movable stage 130 positioned elevationallyadjacent the objective lens (e.g., below the objective lens 128) forsupporting a sample 132 (i.e., photopolymer resist) thereon, and anelectronic control system 134 for controlling at least one of (orpossibly both of) the motion of the movable stage 130 or the objectivelens 128. Optionally a single electronic subsystem (e.g., system 112 orsystem 134) may be used to perform all the control operations for theapparatus 100.

The apparatus 100 differs from the apparatus 10 principally in itsability to continuously monitor one of the non-processing diffractedbeams (i.e., the “mTh diffracted order” beam 111′ in FIG. 2) from thetunable mask 110, and using the beam monitoring power meter 116 tomonitor for changes in the beam power. The beam performing theprocessing, which may be termed the “processing beam” emitted from thetunable mask 110, is designated by reference number 111. The beam powercontrol unit 117 is coupled to the power meter 116 for real-time beamintensity control of the beam incident onto the tunable mask. To controlthe power of the incident beam, the power control unit 117 may control arotating half-wave plate 104 that is followed by a polarizing beamsplitter 106. A polarization-based power control scheme is effectivebecause pulsed laser sources often emit linearly polarized light. Whenpolarization-based power control is implemented, polarization-dependentanisotropy of the multiphoton polymerization process may be minimized byintroducing a quarter-wave plate to convert linearly polarized lightinto circularly polarized light before the light enters the objectivelens. Another power control technique could be to rotate into positionone of several neutral density filters into the path of the processingbeam 111 before entering the objective lens or by introducing thefilters into the path of the beam incident onto the tunable mask 110.

Referring to FIG. 3, a high level diagram is presented to show, inbroader fashion, major subsystems of an apparatus 100′ in accordancewith the present disclosure. The apparatus 100′ makes use of a pulsedlaser beam 102′ which is received by a beam conditioning subsystem 104′.The pulsed laser beam leaves the beam conditioning subsystem 104′ and isreflected from a mirror 108′ to a tunable mask 110′. The tunable mask110′ produces a beam that is directed to a collimator 115′. A collimatedoutput beam from the collimator 115′ is reflected from a mirror 126′toward a focusing element 128′. An imaging system 118′ may be used toimage the focal plane. A control unit 140′ may be used to control thelaser, beam conditioning unit, the imaging system, motion stages, orfocusing elements by receiving feedback signals or synchronizationtrigger signals from the various units connected to the control unit.

Referring briefly to FIG. 6, a high level flowchart 200 is shown ofvarious operations that may be performed by the apparatus 10 or 100 or100′ in carrying out the methodology of the present disclosure. Atoperation 202 a pulsed laser beam is generated. At operation 204 thetunable mask (26 or 110) may be used to digitize the beam (i.e.,discretely pattern subsections of the beam to have high vs lowintensities) and selectively turn on only specific ones of the pixelswithin the mask to create the “processing beam” (i.e., beam 28 or 111)to use in processing a layer of the part (i.e., of the sample 132).Optionally, one of the beams not being used for processing (i.e., the“m^(th) diffracted mode”) may be selected and its power monitored, asindicated at operation 206. Also optionally, if operation 206 isperformed, then at operation 208 the power of the processing beam may beadjusted in real time based on the measured power of the m^(th) beam.

At operation 210 the processing beam may be collimated. At operation 212the collimated processing beam may be used to begin/continue processingan entire layer within or on the sample (i.e., the photopolymer resist)in parallel. At operation 214 the movable stage (50 or 130) and/or thefocusing elements (e.g., the objective lens 44 or 128 and/or theelectrically tunable lens (ETL)) may be controlled as needed during thepolymerization process. At operation 216 a check is made if the presentlayer being processed has been completed, and if not operations 206-216may be repeated. If the check for completion of processing of thecurrent layer at operation 216 produces a “Yes” answer, then a check ismade at operation 218 if the entire sample part is complete (i.e., alllayers processed/formed). If the check at operation 218 produces a“Yes”, answer, then the process ends, but if the check at operation 218produces a “No” answer, then digital information for writing the nextlayer of the part may be obtained, as indicated at operation 220, andoperations 204-216 may be repeated to write out the next layer. Tosynchronize the various components of the system including the tunablemask, the control unit may wait for synchronization signals from themotion stages (for sample or objective) or the camera from the imagingsystem or for trigger signals from internal or external clocks (such asthe pulsed laser itself). Without synchronization, printing in undesiredlocations of the resist material may be observed.

Synchronization of the laser illumination, tunable mask, and the motionstages also enables improving the resolution of the printing processthrough a “super-resolution” printing technique with a sub-pixelresolution. In this technique, the motion stage is moved before theexposure dosage at a spot can exceed the threshold dosage. By moving thestage by only a small amount so as to overlap the point spread function(PSF) of the illumination on top of the previously illuminated spot, itis possible to exceed the threshold dosage in only a fraction of theoverlap region. Thus, this super-resolution printing enables printingfeatures smaller than the features available without moving the stagesduring or between projections. This super-resolution printing can beimplemented along all three axes (X, Y, Z) separately or in combinationwith each other to print finer features. Without any stage motion,super-resolution printing may be implemented by projecting a series ofimages from the tunable mask wherein images are offset from each otherby at least a pixel. To obtain sub-pixel feature resolution, theduration of exposure of these images should be lower than the durationof exposure that corresponds to the threshold exposure dosage at thelaser illumination intensity. When stage motion is combined with thispixel-offset projection technique, super-resolution printing may besimultaneously obtained along multiple axes.

Although a major focus of this disclosure is on printing of submicronfeatures, the systems and methods disclosed herein may also be used toprint larger features on the scale of several micrometers. This may beachieved by simply swapping a high numerical aperture (NA) objectivelens with a low numerical aperture objective lens. As feature sizes aredetermined by the spatio-temporal distribution of light intensity, alow-NA objective lens generates larger features. An advantage of using alow-NA lens is that one could benefit from the low magnification andwider fields of view of such lenses (such as low-NA 10× or 25× lensesversus high-NA 40× or 100× lenses). Such lenses would then significantlyincrease the area of light projection thereby increasing the overallprinting rate by one to two orders of magnitude. Thus, a balance oftradeoff between rate of printing versus feature size resolution can beachieved by a combination of super-resolution printing and properselection of objective lenses. Referring briefly to FIG. 7, a high levelflowchart 300 is shown of various operations that may be performed bythe apparatus 10 or 100 or 100′ in carrying out the methodology of thepresent disclosure for the techniques of grayscale printing orsuper-resolution printing. The flowchart 300 differs from flowchart 200in the ability to change the mask pattern or to move the stage and/orthe focusing elements before writing of a particular layer is complete.This capability is essential to implement grayscale printing andsuper-resolution printing. More specifically, in FIG. 7 operations302-320 correspond to the previously described operations 202-220 ofFIG. 6. However, as noted above, with the methodology shown in FIG. 7,operations 304 and 310-316 (or alternatively operations 304-316) arere-performed if the check at operation 316 indicates that a currentlyprocessed layer is not complete. Re-performing operation 304 enablesfurther control of the digital mask and/or the movable stage, whenneeded, to implement grayscale printing or super-resolution printing.

The various embodiments and methodology of the present disclosuredescribed herein presents a new parallel, two-photon lithographytechnique that ensures depth resolvability on the order of a singlemicron, and an in-plane feature size less than about 350 nm. Arbitrarilycomplex structures may thus be generated by projecting a series ofpatterned “light sheets” that are dynamically tuned through the tunablemask (26 or 110). Although the method described herein may appearfunctionally similar to conventional DMD-based parallelization used inpresent day projection micro-stereolithography systems, the apparatusand method of the present disclosure implements a fundamentallydifferent optical system that ensures that the light sheet (i.e., theprojected image) is both spatially and temporally focused. By overcomingthis barrier to depth resolvability in femtosecond projection optics,the present disclosure successfully increases the scale-up of rate by afactor of 100× while still maintaining the <350 nm feature sizeresolutions of high-quality serial techniques. Thus, the apparatus andmethod of the present disclosure eliminates a fundamental barrier toscaling up submicron additive manufacturing and transforms two-photonlithography into a viable system for high-volume additive manufacturingof functional parts with nanoscale features.

The various embodiments and methodology of the present disclosure areexpected to have a wide range of applicability, for example in 3Dprinting applications in the microelectronics industry, in fabricationof high energy laser targets; in 3D printing applications for printingphotonic crystals (i.e., sensors), in mechanical metamaterials (e.g.,low density, high strength engineered metamaterials), and inmicrofluidics (e.g., for biomedical diagnostic strips), just to name afew examples of potential applications.

While various embodiments have been described, those skilled in the artwill recognize modifications or variations which might be made withoutdeparting from the present disclosure. The examples illustrate thevarious embodiments and are not intended to limit the presentdisclosure. Therefore, the description and claims should be interpretedliberally with only such limitation as is necessary in view of thepertinent prior art.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

What is claimed is:
 1. An apparatus for performing an additivemanufacturing operation to form a structure by processing a photopolymerresist material, the apparatus comprising: a laser source for generatinga laser beam; a tunable mask for receiving the laser beam wherein thetunable mask comprises an optically dispersive element and is configuredto generate a plurality of differing images; a control system configuredto control the tunable mask to generate the plurality of differingimages in sequential fashion; the tunable mask further being configuredto split the laser beam into a plurality of emergent beams, wherein eachsaid emergent beam emerging from the tunable mask comprises asubplurality of beamlets of varying or identical intensity, for each oneof the differing images, and wherein each said beamlet from each saiddiffering image emerges from a unique subsection of illuminated regionsof the tunable mask during projection of each said differing image; acollimator for collecting and collimating at least one of the emergentbeams, from each said differing image, from the tunable mask to form acollimated beam; a power monitoring unit to collect and measure thepower of at least one of said plurality of emergent beams from each saidimage that are not being projected into a focused image plane and notdelivered to the collimator; one or more focusing elements to focus thecollimated beam into a focused beam which is projected as the focusedimage plane on or within the photopolymer resist material, wherein thetunable mask, the collimator, and the focusing elements are so orientedand positioned as to create the same optical path length between thetunable mask and the focused image plane for all optical frequencies ofthe laser beam; and wherein the focused beam simultaneously illuminatesa layer of the photopolymer resist material, the focused beam using thediffering images to generate a cumulative non-linear exposure dosewithin the focused image plane to selectively polymerize portions of thephotopolymer resist material within the focused image plane.
 2. Theapparatus of claim 1, wherein an incident aperture of the collimator islarge enough to collect all wavelengths contained within a single one ofthe emergent beams emerging from the tunable mask but sufficiently smallto block all wavelengths of all other ones of the emergent beams.
 3. Theapparatus of claim 1, wherein the collimator comprises a convex lens ora concave mirror.
 4. The apparatus of claim 1, further comprising amotion stage to support and move the resist material relative to thefocused image plane.
 5. The apparatus of claim 4, further comprising amotion stage to support the one or more focusing elements and to axiallymove the focused image plane toward or away from the resist material. 6.The apparatus of claim 1, wherein at least one of the one or morefocusing elements comprises a focus-tunable optics comprising anelectrically tunable lens (ETL).
 7. The apparatus of claim 1, furthercomprising a power monitoring system to monitor the power of at leastone of the emergent beams emerging from the tunable mask that are notfocused on the resist material.
 8. The apparatus of claim 7, furthercomprising a power control unit including at least one of: a rotatinghalf-wave plate followed by a polarizing beam splitter to control thepower of the beam received by the tunable mask; or a rotating neutraldensity filter wheel to control the power of the beam received by thetunable mask.
 9. The apparatus of claim 1, wherein the tunable maskcomprises a digital micromirror device (DMD).
 10. The apparatus of claim1, further comprising an imaging system using an incoherent opticalsource to monitor the focused beam illuminating the photopolymer resistmaterial.
 11. The apparatus of claim 1, wherein the tunable maskcomprises a spatial light modulator (SLM).
 12. An apparatus forperforming an additive manufacturing operation to form a structure byprocessing a photopolymer resist material, the apparatus comprising: alaser source for generating a pulsed laser beam having a non-uniformGaussian profile; a beam homogenizer configured to receive the pulsedlaser beam and to convert the non-uniform Gaussian profile to a uniformflat-top profile; a tunable mask for receiving the pulsed laser beam,wherein the tunable mask includes a digital micromirror device (DMD)including a plurality of independently controllable pixels that may beturned on or off; a control system for configured to control the tunablemask to generate a plurality of differing images in sequential fashion;the tunable mask further being configured to split the pulsed laser beaminto a plurality of emergent beams, for each one of said plurality ofdiffering images, wherein each said emergent beam emerging from thetunable mask comprises a subplurality of beamlets of varying oridentical intensity, for each one of said plurality of differing images,and wherein each said beamlet emerges from a unique pixel; a collimatorfor collecting and collimating all wavelengths of only a select one ofthe plurality of emergent beams, from each one of said differing images,from the tunable mask, and the collimator being configured to block allwavelengths of all other ones of the emergent beam, to produce acollimated beam; one or more focusing elements to focus the collimatedbeam into a focused beam which is projected as a focused image plane onor within the photopolymer resist material, wherein the tunable mask,the collimator, and the focusing elements are so oriented and positionedas to create the same optical path length between the tunable mask andthe focused image plane for all optical wavelengths of the pulsed laserbeam, and for each one of the differing plurality of images; a powermonitoring unit configured to collect and measure the power of at leastone of said plurality of emergent beams that are not being projectedinto the focused image plane and not delivered to the collimator; amotion stage to support and move the photopolymer resist materialrelative to the focused image plane; and wherein the focused beamsimultaneously illuminates a layer of the photopolymer resist materialusing the plurality of differing images, and in connection with movementof the photopolymer resist material by movement of the motion stage,generates a cumulative non-linear exposure dose within the focused imageplane to selectively polymerize portions of the photopolymer resistmaterial within the focused image plane.
 13. The apparatus of claim 12,wherein the tunable mask is oriented at such an angle to the pulsedlaser beam being received as to generate a blazed grating condition forthe center wavelength of the pulsed laser beam being received.
 14. Theapparatus of claim 12, further comprising a power monitoring unit tocollect and measure the power of at least one of said plurality ofemergent beams that are not used to generate the focused image plane.15. The apparatus of claim 14 further comprising a power control unitincluding one of: a rotating half-wave plate followed by a polarizingbeam splitter; or a rotating neutral density filter wheel to control thepower of the pulsed laser beam received by the tunable mask.
 16. Theapparatus of claim 12, further comprising a control unit to tune the DMDupon receiving a trigger signal from an internal or external clock. 17.The apparatus of claim 12, wherein one of the focusing elementscomprises an electrically tunable lens (ETL) to optically translate anaxial position of a focal plane toward or away from the photopolymerresist material.
 18. The apparatus of claim 17, further comprising acontrol unit to tune the DMD upon receiving synchronization or triggersignals from at least one of: the motion stage supporting thephotopolymer resist material; the ETL; an internal clock; or an externalclock.